Many manufacturers of gas turbine engines are now using advanced investment casting techniques for producing cast metal turbine nozzles or airfoils (e.g., for gas turbine engine blades or vanes) that include intricate air cooling channels to improve efficiency of airfoil cooling. The internal cooling passages are formed in the cast airfoils using one or more complex airfoil shaped ceramic cores positioned in a ceramic shell mold where molten metal is cast in the mold about the core. The ceramic core(s) are responsible for producing internal structural features of the airfoil such as internal cavities and ribs.
A typical ceramic core is made using a plasticized ceramic compound which is injection molded or transfer molded at an elevated temperature in a core die or mold. The core is then hardened by firing or baking. The finished fired core is then positioned within a pattern die cavity in which a fugitive pattern material (e.g., wax or plastic) is introduced about the core to form a core/pattern assembly for use in the well known lost-wax investment casting process. Next, the core/pattern assembly is repeatedly dipped in ceramic slurry, drained of excess slurry, coated with coarse ceramic stucco or sand particles and dried to build up multiple ceramic layers that collectively form a shell mold about the assembly. The pattern then is selectively removed to leave a shell mold with the ceramic core situated therein and molten metal is poured into the mold. After the molten metal solidifies, the mold and core are removed to leave a cast airfoil with one or more internal passages where the core(s) formerly resided.
In the production of hollow metal machine parts, such gas turbine nozzles and airfoils, the above investment casting process is often implemented based upon a free-floating core design. For at least the following among other reasons, as internal geometry designs progress in complexity and incorporate more of the overall 3-D airfoil or nozzle shape, the casting must be able to be “balanced” so as to allow for an optimum fit of the internal geometry to the primary datum scheme of the part thus requiring the core to be a “free-floating” element in the design. However, the use of a free-floating core design causes problems during subsequent production machining of the part. In particular, the use of a free-floating core design results in a certain amount of positional variation of the cast internal features about the fixed external datum structure of the part. Such variation is highly undesirable when attempting to perform accurate gauging or precision machining operations on these core-produced internal features.
Generally, it is only possible to use wall thickness and external layout sections in relation to fixed external datums comprising a primary datum scheme to approximate the position of the internal geometry of a particular cast airfoil/nozzle part. Because of this, automated machining of internal core-produced features is often inaccurate, if not unfeasible. This is due, at least in part, to the fact that conventional automated machining methods rely upon a part's fixed external datum scheme/structure for locating and/or holding a part during machining/gauging operations and this fixed “primary” datum scheme is inaccurate with respect to internal core-produced cast features due to positional variations of the features caused by the use of a free-floating core. (Conventional commercially available packaged-software applications that are used for controlling most automated gauging and machining equipment typically use this fixed external datum scheme and compute a best fit determination to all datums for an particular part.) In present day complex airfoil designs, a gas turbine airfoil shape must allow for a “best fit” of external airfoil features in such a manner as to achieve and optimize a particular desired turbine throat area. Internal features of the airfoil are generated by utilizing a core during the casting process. The core can float, twist, shift, etc. relative to the external airfoil geometry during the casting process. This movement of the core causes the internal core produced features to be placed in an unknown position relative to the external airfoil shape. Many of these internal core produced features require precise machining to allow for fit up and/or attachment of other components by welding or brazing. Very tight machining tolerances are required to maintain a precise fit or a fit that will allow for successful brazing and or welding of the attached components. If these internal core produced features, which have moved relative to the external features during the casting process, were machined based on fixtureing to the external features, the machining tolerances would be excessive. Consequently, a need exists for a method and/or arrangement for determining the location of the core produced geometry such that the resulting internal core-produced features of the casting may be machined relative to core position not to the external airfoil features.